The present invention relates to a photomask blank and a photomask for typical use in the microfabrication of high density semiconductor integrated circuits such as LSI and VLSI, color filters for charge coupled devices (CCD) and liquid crystal devices (LCD), and magnetic heads. It also relates to a method of manufacturing the same.
Photolithography using photomasks is employed for the microfabrication of high density semiconductor integrated circuits such as LSI and VLSI, color filters for charge coupled devices (CCD) and liquid crystal devices (LCD), magnetic heads, and the like.
In general, the photomask is prepared from a photomask blank in which a light-shielding film typically in the form of a chromium base film is formed on a transparent substrate of quartz glass or aluminosilicate glass as by sputtering or vacuum evaporation. A predetermined pattern is formed in the chromium base light-shielding film, resulting in the photomask.
JP-B 62-37385 discloses a photomask blank comprising on a substrate a chromium film serving as the light-shielding film wherein CrO is formed on the surface of the chromium film for antireflection purposes and another antireflection film is formed on the substrate side. There were proposed a variety of antireflection films including CrON (JP-B 61-46821 and JP-B 62-32782) and CrN (JP-B 62-27386 and JP-B 62-27387). Also JP-B 4-1339 discloses a single layer film of CrN and JP-B 62-37384 discloses a multilayer film of Cr+CrN+CrON.
For further microfabrication purposes, a method of processing to a finer pattern using a phase shift film that utilizes optical interference is practiced. For example, JP-A 7-140635 uses an oxygen-doped chromium film or molybdenum silicide film alone as the phase shift film. JP-B 62-59296 discloses a combination of a light-shielding film with a phase shift film. JP-A 62-18560 discloses the use of chromium alone or chromium in admixture with at least one of oxygen, nitrogen and carbon as the film material. JP-A 7-43888 discloses film deposition by reactive sputtering using oxygen and CH4 as the reactive gas.
In the prior art of photomask blanks and photomasks, it is thus known to add oxygen to a chromium film and to form an oxygen-containing chromium film on a light-shielding film. The oxygen-containing chromium film is usually prepared by introducing oxygen gas as the oxygen source into a deposition chamber. The use of oxygen gas as the oxygen source, however, has the problem that optical properties such as transmittance, reflectance and refractive index tend to vary within the plane of substrate. The use of nitrogen monoxide gas as the oxygen source is also proposed in JP-B 62-37385, but suffers from the same problem.
The use of oxygen gas as the oxygen source also has the problem of low production yield in that because of the high reactivity of oxygen gas, optical properties are sensitive to variations of gas flow rate, which precludes consistent mass production. An attempt to control film properties encounters difficulty, because the film quickly becomes an oxide film when the oxygen flow rate exceeds a certain level. Then the control of film quality becomes impossible and the rate of deposition rapidly declines.
An object of the invention is to provide a photomask blank and a photomask of quality which have high uniformity of optical properties within the plane of substrate and can be consistently manufactured because of ease of control of deposition of a chromium base film. Another object of the invention is to provide methods for preparing the photomask blank and the photomask.
It has been found that when reactive sputtering is effected using chromium or chromium containing at least one element of oxygen, nitrogen and carbon as the target and a mixture of carbon dioxide gas and an inert gas as the sputtering gas, a chromium base film is formed on a substrate to high uniformity of optical properties within the plane of the substrate. The deposition of the chromium base film under these conditions ensures ease of control and consistent mass production. Specifically, a chromium base film of chromium oxycarbide (CrCO) or chromium oxynitride carbide (CrCON) can be formed to high quality, effectively overcoming the problems of the prior art.
In one aspect, the invention provides a photomask blank comprising a transparent substrate and at least one layer of chromium base film thereon, wherein the chromium base film has been formed by reactive sputtering using chromium or chromium containing at least one element of oxygen, nitrogen and carbon as the target and a sputtering gas containing at least carbon dioxide gas and an inert gas. The chromium base film is preferably comprised of chromium oxycarbide (CrCO) or chromium oxynitride carbide (CrCON). The invention also provides a photomask manufactured by lithographically patterning the photomask blank.
In another aspect, the invention provides a method of manufacturing a photomask blank having at least one layer of chromium base film on a transparent substrate, comprising the step of effecting reactive sputtering using chromium or chromium containing at least one element of oxygen, nitrogen and carbon as the target and a sputtering gas containing at least carbon dioxide gas and an inert gas, for forming the chromium base film. The chromium base film is preferably comprised of chromium oxycarbide (CrCO) or chromium oxynitride carbide (CrCON). By lithographically patterning the photomask blank, a photomask is manufactured.
When a chromium base oxide film is deposited on a transparent substrate by a reactive sputtering technique, the invention recommends to use carbon dioxide gas as the oxygen source. Since carbon dioxide is less reactive than oxygen, the gas can uniformly spread and distribute in the chamber over its full extent so that the chromium base film being deposited becomes of uniform quality.
The film deposition by sputtering using carbon dioxide gas as the oxygen source can minimize the non-uniformity of optical properties within the substrate plane for the following reason. In the process of flowing oxygen or another reactive gas capable of reacting with chromium into a deposition chamber from its inlet, variations of reflectance arise because oxygen is successively consumed for oxidation from a position nearer to the inlet, that is, the degree of oxidation is higher at a position nearer to the inlet. When oxygen is fed from without the target, oxygen is consumed on the outside of the target so that the concentration of oxygen lowers toward the inside, resulting in the degree of oxidation varying within the plane to produce a distribution of optical constant.
In contrast, the invention uses carbon dioxide gas as the oxygen source in reactive sputtering. Since the low reactivity carbon dioxide gas is little consumed until it is activated by a plasma, it can uniformly spread around in the chamber so that a film being deposited may have a more uniform degree of oxidation. As a result, the uniformity of optical properties within the substrate plane are drastically improved. Film properties can be controlled by using both carbon dioxide gas and an inert gas as the sputtering gas and adjusting the mixing ratio therebetween.
The control of film composition is facilitated by the simultaneous use of carbon dioxide gas and an inert gas as the sputtering gas. Also because of the low reactivity, the carbon dioxide gas allows for a greater margin against unexpected variations of many parameters associated with the deposition process. Then the chromium base film can be deposited in a stable and controllable manner without decreasing a deposition rate. Also advantageously, reflectance can be reduced.